1. Field of the Invention
The present invention relates generally to a method of reading a radiation image converting panel, and more particularly to a radiation image-converting panel reading method of a double focusing type that takes advantage of the photostimulated luminescence of a stimulatable fluorescent substance (BaFBr, Eu2+).
2. Description of the Related Art
A radiation image recording-reproducing method (radiation image converting method) employing a stimulatable fluorescent substance is known as a replacement method for radiography that employs a combination of radiographic film and a sensitizing screen. This method makes use of a radiation image converting panel that contains a stimulatable fluorescent substance (also stated as a stimulatable fluorescent substance sheet). In the method, radiation transmitted through or emitted from a subject is absorbed in the stimulatable fluorescent substance contained in the sheet. Then, an electromagnetic wave (excitation light), such as visible light, infrared radiation, etc., is irradiated to the stimulatable fluorescent substance to excite it. With the excitation, the radiation energy that has been stored in the stimulatable fluorescent substance is emitted as fluorescent light. This phenomenon is called photostimulated luminescence. The fluorescent light is photoelectrically read and converted to an electrical signal. Based on the electrical signal, the radiation image of the subject is reproduced as a visible image. After the radiation energy remaining in the stimulatable fluorescent substance has been erased, the radiation image converting panel that has finished the reading is repeatedly used by the same radiation recording-reproducing method.
The radiation image converting panel employed in the above-mentioned radiation image recording-reproducing method is normally provided on its lower surface with a supporting member and on its upper surface with a protective film. The stimulatable fluorescent layer of the radiation image converting panel is usually made up of stimulatable fluorescent particles and a bonding agent containing and supporting the fluorescent particles in a dispersed state. However, a stimulatable fluorescent layer consisting of an aggregate of a stimulatable fluorescent substance without a bonding agent formed by a deposition or sintering method, or a stimulatable fluorescent layer containing a high polymer in a gap in the above-mentioned aggregate, is also known. Radiation image converting panels employing these stimulatable fluorescent layers can all be used in the aforementioned radiation image recording-reproducing method.
Reading of radiation image information by the radiation recording-reproducing method is generally performed by irradiating excitation light to the upper surface of the radiation image converting panel, then reading out the fluorescent light emitted from the stimulatable fluorescent substance by a focusing guide provided on the side where the excitation light is irradiated, and converting the read fluorescent light to an electrical image signal (single focusing type). However, in the case where the fluorescent light emitted from the stimulatable fluorescent substance is read out as much as possible, or in the case where, for a latent image formed by radiation energy stored within the radiation image converting panel, an energy intensity change (intensity distribution) in the direction of depth of the sheet is obtained as radiation image information, a double focusing type that focuses the fluorescent light emitted from the upper and lower surfaces of the radiation image converting panel is utilized. A radiation image recording-reproducing method of this double focusing type is described, for example, in Japanese Unexamined Patent Publication No. 55(1980)-87970.
In the radiation image recording-reproducing method of the above-mentioned single focusing type or double focusing type, it is desirable that the radiation image converting panel have high sensitivity and be capable of reproducing a high-quality radiation image. Particularly, in the formation of a medical radiation image that employs X-rays, which is a representative use of the radiation image recording-reproducing method, it is desirable to obtain a radiation image having high quality (particularly, high sharpness related with high resolution), with a small exposure-dose of X-rays.
The diffusion of excitation light within the radiation image converting panel has a significant influence on the sharpness of a radiation image formed by the radiation image recording-reproducing method. This is for the following reason. The latent image of the radiation energy recorded on the radiation image converting panel is read out by moving a beam of excitation light to irradiate it to the panel surface in a time-series manner and then sequentially focusing the fluorescent light emitted from the panel surface by the irradiation of the excitation light. However, if the irradiated excitation light diffuses within the panel (particularly, in the plane direction), the excitation light will go beyond the irradiated region and excite the fluorescent particles outside the irradiated region that have radiation energy, and consequently, the radiation energy outside the irradiated region, as well as the radiation energy inside the irradiated region, will be read out as fluorescent light.
It is known that, in the radiation image converting panel employed in the radiation image recording-reproducing method of the single focusing type, excitation-light reflecting partitions for dividing the panel finely along the panel surface are provided in the stimulatable fluorescent layer of the panel to avoid diffusion of excitation light. For example, Japanese Unexamined Patent Publication No. 59(1984)-202100 discloses that a honeycomb structure consisting of cells divided by partitions is provided in a radiation image converting panel wherein a stimulatable fluorescent layer is provided on a supporting member and that each cell is filled with a stimulatable fluorescent substance. Japanese Unexamined Patent Publication No. 62(1987)-36599 discloses a radiation image converting panel wherein a large number of recesses (in which the ratio between the diameter and the depth is 1:3.5 or greater) are regularly provided on one surface of a supporting member and filled with a stimulatable fluorescent substance. Japanese Unexamined Patent Publication No. 2(1990)-129600 discloses a radiation image converting panel wherein a great number of holes formed in the direction of depth of a supporting plate are filled with a stimulatable fluorescent substance. Japanese Unexamined Patent Publication No. 2(1990)-280100 discloses a radiation image converting panel wherein a micro structure in the form of a honeycomb, formed on a supporting member, is filled with a fluorescent substance. PCT Japanese Publication No. 5(1993)-512636 discloses a method of fabricating phosphorescent pixels by the use of a metal mold.
The above-mentioned stimulatable fluorescent layer, in which a large number of recesses formed in the base or supporting member are filled with fluorescent particles, is effective in forming a radiation image with high quality (particularly high sharpness), because diffusion of excitation light is prevented by the supporting material which becomes partitions within the radiation image converting sheet. However, since the partitions occupy part of the stimulatable fluorescent layer, the problem of the fill amount of the fluorescent particles per unit volume being necessarily reduced will arise. A reduction in the amount of the fluorescent substance within the stimulatable fluorescent layer per unit volume reduces an absorption amount of X-rays and therefore gives rise to a reduction in the sensitivity of the radiation image converting panel. The sensitivity of the radiation image converting panel can be increased by increasing the thickness of the layer. An increase in the layer thickness, however, results in a reduction in the sharpness of an image.
As a means for solving such a problem, in Japanese Unexamined Patent Publication No. 7(1995)-27078 there is disclosed a radiation image information reader employing a radiation image converting panel wherein a stimulatable fluorescent substance is deposited by a deposition method and has a pillar-shaped crystal structure. Since the fluorescent layer formed by deposition contains no bonding agent, it becomes possible to make the density of the fluorescence substance higher, compared with a dispersion system containing a bonding agent. This enhances the radiation absorption factor and the sensitivity with respect to radiation, resulting in an enhancement in the graininess of an image.
On the other hand, in the radiation image information reader that is employed in a radiation image recording-reproducing system, from the viewpoint of shortening the time required to read fluorescent light, device compactness, and cost reduction, a line light source, which irradiates excitation light in line form to the sheet, such as a fluorescent lamp, a cold cathode fluorescent lamp, a light-emitting diode (LED) array, a broad area laser, etc., is used as an excitation light source, and a line sensor, wherein a large number of photoelectric converting elements are arrayed along the direction of length of the line excitation light irradiated to the sheet by the line light source, is used as photoelectric reading means. The radiation image information reader is also provided with scan means movable with respect to the above-mentioned line light source and line sensor in a direction substantially perpendicular to the length direction of the line excitation light (Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, and 1(1989)-101540, etc.).
It is desired that in the radiation image-converting panel reading method of the double focusing type, as with the radiation image-converting panel reading method of the single focusing type, high sensitivity and a high-sharpness radiation image be obtained. Therefore, there is an increasingly strong demand for the development of a radiation image converting panel which is capable of reproducing a radiation image with even higher sensitivity and high sharpness.
In addition, the quantum noise of radiation is contained in the step of storing and recording a radiation image on the stimulatable fluorescent sheet and is superposed on an image signal read out by the radiation image converting panel, so a reading method capable of effectively removing the quantum noise is also required in order to enhance the signal-to-noise (S/N) ratio of a radiation image reproduced by the read image signal.
The present invention has been made in view of the aforementioned problems found in the prior art. Accordingly, it is an object of the present invention to provide a radiation image converting panel which enhances the graininess of an image by increasing the filling density of a fluorescent substance, while reducing scattering of excitation light by a radiation image converting-panel reading method of the double focusing type. Another object of the present invention is to provide a radiation image-converting panel reading method which is capable of collecting a larger amount of image information and enhancing the sharpness of an image, by employing the aforementioned radiation image converting panel.
To achieve the aforementioned objects of the present invention, there is provided a method of reading a radiation image converting panel which comprises at least an optically transparent supporting member and a fluorescent layer having pillar-shaped stimulatable fluorescent substances formed on the supporting member by a deposition method, the reading method comprising the steps of:
exciting the stimulatable fluorescent substances by irradiating excitation light to a surface of the radiation image converting panel on which energy of radiation has been stored and recorded by irradiation of the radiation;
emitting the radiation energy as photostimulated fluorescent light; and
reading out the photostimulated fluorescent light photoelectrically from both surfaces of the radiation image converting panel.
The aforementioned deposition method not only means a vapor deposition method wherein a stimulatable fluorescent substance is evaporated by heating and deposited on a predetermined base, but broadly means a method of depositing a stimulatable fluorescent substance. For example, it includes sputter deposition, chemical vapor deposition (CVD), ion plating, etc.
The expression xe2x80x9cfluorescent layer having pillar-shaped stimulatable fluorescent substancesxe2x80x9d refers to a layer that has pillar-shaped stimulatable fluorescent substances in a direction approximately perpendicular to the surface of the fluorescent layer, the pillar-shaped stimulatable fluorescent substances being optically independent of one another. The words xe2x80x9coptically independentxe2x80x9d mean that the diffusion of irradiated excitation light within the fluorescent layer (particularly diffusion in the plane direction) is suppressed and that the irradiated excitation light hardly goes beyond a region irradiated and excites fluorescent particles having radiation energy outside the irradiated region. That is, the words xe2x80x9coptically independentxe2x80x9d mean that radiation energy outside the irradiated region is difficult to read out as photostimulated fluorescent light, along with radiation energy inside the irradiated region. The pillar-shaped fluorescent substances can be formed, for example, by partitions that divide the fluorescent layer along the plane direction. However, it is desirable that the partitions have a stimulatable fluorescent substance.
Since the radiation image converting panel is a panel comprising at least the supporting member and the fluorescent layer, the radiation image converting panel may be provided with other layers such as a protective layer, a bonding layer, etc. It is preferable that a multi-layer film filter for transmitting only photostimulated fluorescent light be provided on the opposite side from the side of the radiation image converting panel to which the excitation light is irradiated.
It is desirable that the irradiation of the excitation light be performed by a line light source which emits the excitation light in line form. It is also desirable that the reading from both surfaces of the radiation image converting panel be performed by a line sensor having a large number of photoelectric converting elements arrayed in line form, or an area sensor in which a great number of line-shaped photoelectric converting elements are arrayed in rows.
In the radiation image-converting panel reading method of the present invention, the stimulatable fluorescent layer of the radiation image converting panel is constructed of the optically independent pillar-shaped fluorescent substances, formed by a deposition method. Therefore, the directivities of the photosimulated excitation light and photosimulated fluorescent light are enhanced and the permeabilities for the photosimulated excitation light and photosimulated fluorescent light become higher. As a result, the stimulatable fluorescent layer can be made thicker than that formed by the aforementioned conventional coating methods and can have even higher sensitivity with respect to radiation.
Since the stimulatable fluorescent layer formed by the deposition method contains no bonding agent, the density of the fluorescent substance can be made about two times higher, compared with a dispersion system containing a bonding agent. Also, as the radiation absorption factor of the stimulatable fluorescent layer per unit volume can be enhanced, the stimulatable fluorescent layer has high sensitivity with respect to radiation and is capable of enhancing the graininess of an image. Furthermore, because the stimulatable fluorescent layer is constructed with the optically independent pillar-shaped stimulatable fluorescent substances, the stimulatable fluorescent layer is capable of enhancing the sharpness of an image by reducing scattering of excitation light.
If the partitions for dividing the optically independent pillar-shaped fluorescent substances, formed by the deposition method, contain a stimulatable fluorescent substance, the partitions can suppress a reduction in the sensitivity of the radiation image converting panel due to a reduction in an absorption amount of X-rays, without giving rise to the problem of the fill amount of the fluorescent particles of the fluorescent layer per unit volume being reduced.
In addition, the radiation image-converting panel reading method of the present invention can capture a larger amount of photostimulated fluorescent light in the direction of thickness of the fluorescent layer, because it photoelectrically reads out photostimulated florescent light from both surfaces of the radiation image converting panel. Therefore, the reading method is capable of enhancing quality (sharpness) and also enhancing the S/N ratio of the radiation image reproduced by an image signal obtained. In the case where a multi-layer film filter for transmitting only photostimulated fluorescent light is provided on the opposite side from the side of the radiation image converting panel to which the excitation light is irradiated, the multi-layer film filter selectively transmits only the photostimulated fluorescent light emitted from the radiation image converting panel. Therefore, the reading method is capable of further enhancing the S/N ratio of the radiation image reproduced by an image signal obtained.
Furthermore, by performing the irradiation of excitation light by a line light source for irradiating excitation light in line form, and by performing reading from both surfaces of the radiation image converting panel using a line sensor having a large number of photoelectric converting elements arrayed in line form, the time for reading photostimulated fluorescent light can be shortened, and furthermore, the converting panel can be made compact and the number of scanning components reduced.